Most energetic stellar collisions in the Universe

September 01, 2023

In dense stellar environments, stars can collide. If there is a massive black hole nearby – at the centre of galaxies – these collisions can be so energetic that the two stars are completely destroyed upon collision, leaving behind an expanding gas cloud. While the collision itself can generate a very luminous flare for several days, there might be an even brighter flare that can last up to many months, as the gas cloud is captured by the nearby black hole. A research team led by MPA has estimated the observables of such powerful events for the first time using the two state-of-the-art codes AREPO and MESA, developed at MPA.

What are the most energetic collisions between stars in the Universe? Such collisions would happen if the stars move at high relative velocities. In the deep potential well of the massive black hole at the centre of a galaxy, stars can reach a few percent of the speed of light (up to 10 000km/s). The collision of two such fast-moving stars would be fascinating to observe, because the resulting flare could be at least as luminous as various types of electromagnetic transients, such as tidal disruption events or supernovae.

Because we did not understand their observational signatures, however, not much effort has been spent searching for these high-velocity collisions. A research team led by an MPA fellow has now made quantitative predictions how such black hole-driven destructive collisions between giant stars could be observed. For their analysis, the team used the state-of-the-art simulation codes AREPO and MESA.

Collision of fast red giants

This animation shows the collision of two red giant stars with large relative velocity. The time starts about one day before the event and runs until 30 days after. The colour scale shows the density of the material, the two red dots indicate the locations of the cores. Note the changing length scale (depicted as solar radii), which first decreases and then increases.

In particular, the team analysed two red giant stars, colliding at velocities much greater than the escape velocity of the colliding stars. This means that the two stars are entirely destroyed. Very powerful shocks convert a large fraction of the initial kinetic energy into heat, driving the resulting gas cloud to expand quasi-spherically.

The maximum expansion speed of the cloud is larger than the initial relative velocity of the stars, and the parameters of the gas cloud depend rather strongly on the collision velocity. A collision between larger stars colliding at a higher speed tends to result in greater conversion efficiency. As the heat energy escapes from the cloud, a prompt flare with a peak luminosity comparable to that of a supernova explosion (1041 - 1044 erg/s) can be generated. Because of the rapid expansion of the cloud, the prompt flare becomes very faint in days or a week.

However, the expanding gas cloud interacts with the nearby black hole. The accretion of the gravitationally captured gas creates a second flare that could even be brighter and lasting much longer than the first flare. This heightened luminosity can be sustained for up to ten years.

These unique features of the electromagnetic radiation make such events a promising probe for the existence of dormant black holes. In addition, the growth of black holes through the accretion of the collision products would be another venue for the growth mechanism for seed black holes at high redshifts.

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